The invention described herein was made in the course of work under a grant award No. HL 19019 from the Department of Health, Education, and Welfare.
The present invention relates, in general, to the measurement of the volume flow of fluids through conduits, and more particularly to the measurement of the flow of blood through arteries and veins.
The technique of utilizing wave signals such as light or sound for measuring the velocity of flow of a fluid which is transparent to the wave is well known. For example, the use of light in such measurements was established in a fundamental physical experiment in 1851, where two coherent light beams were simultaneously projected upstream and downstream of a flowing liquid. A light interference pattern was produced, with the position of maxima and minima reflecting the velocity of the flowing medium. More recently, acoustic waves in the ultrasonic frequency range have been used to measure the flow velocity of liquids and gases that are acoustically transparent. In this type of measurement, a transmitter and a receiver, which may be transducers constructed from a piezoelectric material, are positioned in such a way that the transit time of the sound wave between the transmitter and the receiver is affected by a component of the flow of the medium being measured. The transit time of a sound wave traveling between the two transducers is a function of the relative average velocity of the second conductive medium times the acoustic path length. If it is desired to obtain the average velocity from this transit time measurement, the distance over which the velocity is averaged must be measured.
The method of using acoustic waves for measuring the flow velocity in liquids and gases has been implemented for biomedical use, with intravascular systems being provided to utilize the difference in transit time between an upstream and a downstream projected burst of sound to measure flow velocity with a known zero reference, and a similar scheme has been used for an extra-vascular flow measurement device. However, because it is necessary to obtain a measurement of the path length followed by the waves in transversing the conduit carrying the fluid to be measured in order to deduct average velocity, such devices have presented problems not only in general measurement applications, but particularly in the measurement of blood flow, for it is often extremely difficult to obtain an accurate measurement of the inside diameter of a conduit such as, for example, a blood vessel. Further, if it is desired to obtain volume flow data, it is necessary to know with great accuracy both the vessel geometry and the relationship between sample average velocity and total average velocity, which is a function of the flow profile across the diameter of the conduit, and again, such data is usually very difficult to obtain, particularly in blood vessel measurements. Any error in this dimension produces a proportional error in the volume flow measurement.
Another source of error in prior ultrasonic flow measuring devices is the fact that the sound waves produced by one transducer may be reflected by the vessel walls and produce false signals in the receiving transducer. Where the medium to be measured is flowing through an accessible conduit which has easily measurable dimensions, many of these problems can be avoided, but where the flow is through a flexible conduit of uncertain internal diameter, such as a blood vessel, additional errors have been produced in prior systems. But even where the conduit is accessible, minor variations in size can produce errors, as can misalignment of the conduit with respect to the transducers. The numerous sources of error have made prior measurements of fluid flow volume inaccurate and unreliable.